Many types of modulators are used to print or display text and images. The desired characteristics of modulators typically include high resolution, high display speed, and freedom from image distortion. Modulators typically include an array of modulator elements which operate in concert to produce an image. Each operation cycle of the modulator is called a modulator cycle. Each image is typically represented as a two-dimensional array of picture elements, or pixels, called a pixel grid. Each pixel represents the smallest independent picture element that may be produced by a modulator. Each pixel may be produced by a single modulator cycle, for example a single LED element that outputs a pulse of variable intensity light to illuminate a pixel, or the pixel may be produced by multiple modulator cycles, for example an LED element that outputs a series of light pulses which act in cooperation to illuminate a pixel. Although each pixel is typically related to a single modulator element, some systems use the output from multiple modulator elements to display one pixel. A modulator element may be an individual LED, an LCD cell, an ink-jet, a digital micromirror device (DMD), a thermal printer head, or an electron gun, or any other device capable of producing an image.
Complete images may be produced in three ways. Sequential scanning, as employed in CRTs and laser printers, requires only one modulator element which sequentially scans the image pixels in a line, one line at a time until the entire image has been scanned. A second method, used by line printers and some image displays, requires at least one modulator element for each image pixel in a line. The modulator elements display all of the pixels in a line simultaneously and sequentially display each line of the image. The third method, used by frame-addressed spatial light modulators (SLMs), uses an array of pixels simultaneously to display an entire image "frame." Frame addressed modulators require a modulator with at least as many elements as the number of image pixels.
High resolution printers require a large number of image pixels to be printed on each line. For example, an electrostatic printer capable of printing 300 pixels or dots per inch (DPI), must print 2550 pixels across one line on 8.5 inch paper. If the image is printed a line at a time, the modulator used in the printer must have at least 2550 elements. As the number of modulator elements increases, so does the complexity and size of the modulator required to generate the image.
The torsion beam digital micromirror device (DMD) SLM, as taught in commonly assigned U.S. Pat. No. 5,061,049, may be used to modulate light in an electrostatic printer. DMDs are manufactured using semiconductor processing techniques and may be fabricated with one million or more modulator elements on a single DMD. However, a DMD with 2550 mirrors in a single row is a large device by semiconductor standards and presents many fabrication challenges. If each pixel of the DMD is 34 .mu.m wide, a row of 2550 pixels is about 3.4 inches long and the die for each device may be approximately 3.5 inches long. This large die size can result in a low wafer utilization. For example, if the 3.5 inch long DMD in the above example is 0.25 inches wide, only 42 DMDs could be manufactured from an eight inch diameter wafer. This would result in a wafer utilization of approximately 73%.
Furthermore, the yield of good DMDs will be low because the large number of mechanical and electrical structures fabricated on each DMD increases the probability of there being at least one defective structure on the DMD. This is partially due to the fact that for a given surface contamination rate, an increase in size of each DMD will increase the probability of a surface contaminant being located on the device. For example, if the probability of a defect in a given structure is 1%, then the probability of producing a defect-free device that includes ten of the structures is about 90%, while the probability of producing a defect-free device that includes 1000 of the structures is about 0.0043%. Because the same process steps, and approximately the same amount of raw materials are used regardless of the number of DMDs formed on a wafer, the cost of processing a wafer is practically independent of the number of DMDs formed on the wafer. Therefore, the low yield and low wafer utilization that occur when large DMDs are manufactured result in a dramatic increase in the cost of good DMDs.
In order to reduce the cost and complexity of the modulators used in image displays, several discrete modulators have been used, each displaying only a portion of the entire image. For example, four DMDs, each with 2,400 elements, are used in place of the 9,600 element DMD in the above example. While this reduces the size of each DMD to about 1.6 inches, and increases both the wafer utilization and device yield, using multiple modulators introduces the possibility of misaligned modulators. If the modulators are not correctly aligned, the resultant image may be distorted, appearing to have features that are not part of the desired image. For example, areas of the image may appear brighter or darker than desired. These image defects are called "artifacts" because they represent an artificial feature caused by the method of image creation instead of a true feature of the desired image.
The effect of artifacts caused by misalignment is increased where one row of modulator elements sequentially produces image pixels line-by-line. This is because misalignment-caused artifacts are duplicated on each line, creating a strong vertical or horizontal feature which is easily perceived by the human eye. Precisely aligning multiple modulators during assembly of the modulator eliminates alignment artifacts but typically requires the use of precision machined surfaces acting as positional references or complex alignment routines. These methods can be expensive to implement and may result in an unacceptably high rework or rejection rate. Thus, there is a need for a system and method of operation that are capable of tolerating some misalignment without the image being deleteriously affected by artifacts.